The invention relates generally to frequency detection circuits, and more particularly to circuits for detecting the presence of transmitted data in the form of Frequency Shift Key (FSK) data.
It is highly advantageous to utilize the telephone system as a transmission medium for communicating FSK-type data. Such data, having frequencies falling within the voice band, is easily accommodated by telephone switching systems.
While data information transmission over land lines is as susceptible to noise, transient voltages and power line AC interferences as voice signals, the need to reproduce the data signals is significantly more critical. This is particularly true because the ramifications of the absence or addition of a data pulse in a data transmission may render the data wholly invalid. This is not the case with voice signal transmission.
It is a general practice of FSK data transmission to utilize a data operated switch at the receiving end to establish a standard by which data information can be distinguished from electrical noise, spikes, or the like. Prior art methods of establishing such a standard consist of detecting the amplitude of the signals appearing on the line. Electrical energy, i.e. amplitudes, above a predetermined level would be considered as data information. Those signals falling below such a level, whether data information or noise, would be disregarded.
A more accurate approach employs a filter which filters the electrical energy to remove unwanted frequencies, and then makes a similar amplitude level comparison to determine the energy content of the signal.
The drawback of both such approaches is that large amplitude transient energy spikes pose a problem insofar as the spikes can rise to the data information amplitude and cause the filter to "ring" at a frequency within the filter bandwidth thereby appearing as data information.